Notes

That one can for example make a mold that will produce copies of a shape has been known since antiquity (see note above). The cybernetics movement highlighted the question of what it takes for self-reproduction to occur autonomously, and in 1952 John von Neumann designed an elaborate 2D cellular automaton that would automatically make a copy of its initial configuration of cells (see page 876). In the course of the 1950s suggestions of several increasingly simple mechanical systems capable of self-reproduction were made—notably by Lionel Penrose. The phenomenon in the main text was noticed around 1961 by Edward Fredkin (see page 877). But while it shows some of the essence of self-reproduction, it lacks many of the more elaborate features common in biological self-reproduction. In the 1980s, however, such features were nevertheless surprisingly often present in computer viruses and worms. (See also page 1092.)